Afraid don't have any personal experience but have spoken to parents of boys who joined in year 9 and who seemed to cope fine.
I would be led by the child's feelings at this stage as well. If they are doing very well (and one does need to consider that being at the top of some comprehensive schools doesnt atutomatically mean they will get top grades at GCSE) and are happliy settled then it might not be worth putting them through another test situation (I believe that there is still stiff competition so success is not guaranteed) and uprooting them.
It also might depend a bit on the child's personality - the first few months could be tricky as they settle into new languages and playing lots of rugby! The school is set up to help them but they would need to be Ok with having to 'catch up' in some areas and perhaps needing a bit of extra help.
In a similar vein - does anyone know if there are any tutors in Colchester who specialise in getting boys ready for the entrance test? (probably best to PM if you have specific names, although not necessarily recommendations, just any info.) Many thanks.